Optical Buffering Methods, Apparatus, and Systems for Increasing the Repetition Rate of Tunable Light Sources

ABSTRACT

In one embodiment, the invention relates to an apparatus for increasing the repetition rate in a light source. The apparatus includes a first optical coupler comprising a first arm, a second arm and a third arm; a first mirror in optical communication with the second arm of the first optical coupler; and a first optical delay line having a first end in optical communication with the third arm of the first optical coupler and a second end in optical communication with a second mirror, wherein light entering the first arm of the first optical coupler leaves the first arm of the first optical coupler either delayed by an amount (τ) or substantially undelayed.

FIELD OF INVENTION

This invention relates generally to the field of frequency-tunable lightsources and more specifically to their application in opticalmeasurement and imaging systems.

BACKGROUND

New applications of measurement and imaging systems that employ tunablelight sources demand increasingly higher data acquisition rates. Forexample, although the latest generation of frequency-domain opticalcoherence tomography (OCT) systems for imaging of coronary arteriesacquire image data at 50,000 lines/sec, even faster speeds are necessaryto scan the full lengths of the major coronary branches. Scanningspectrometers must also operate at higher speeds to permit rapidanalysis of atmospheric aerosols and contaminants over large areas,hyperspectral imaging of moving targets, detection of chemicalsubstances in multiple samples, and other applications.

One of the major impediments to higher speed operation offrequency-domain systems, especially systems that operate continuouslyover a wide spectral range, is the sweep repetition rate of the lightsource. Many high-speed wavelength-swept light sources employmechanically actuated filters that are excited by a substantiallyperiodic waveform with a frequency close to the resonance of the filter.When operated at very high speeds, many tunable light sources produce ausable output over less than 50% of the sweep period of the filter. Thisoccurs because the time required to build up stimulated emission limitsthe available optical gain within the filter bandpass. Even if thebuild-up time of the gain element does not limit the scanning speed fora given application, the non-linearities of the gain element can broadenthe line width of the source. Such undesirable broadening can occurduring either the up sweep (the portion of the sweep over which thewavelength increases) or the down sweep (the portion of the sweep overwhich the wavelength decreases), such that a significant portion of thesweep becomes unusable. Moreover, to ease the demands on the dataacquisition system, it is often desirable to acquire data only duringthe portion of the sweep when the rate of change of the opticalfrequency is approximately constant. These factors impose an upper limiton the effective duty cycle of continuously swept light sources.

A method for increasing the repetition rate of a light source usingtime-multiplexing was first introduced in the context of the design ofFourier-domain mode-locked lasers. This method, referred to here as‘optical buffering’, is based on combining the output of the swept laserwith a delayed version of the swept laser output, which is insertedwithin the interval between laser sweeps during the time when the lasergain medium is turned off or is inactive.

In a conventional buffer, the delayed version of the laser output isgenerated by splitting off a fixed fraction (˜50%) of the laser outputwith an optical coupler, storing the laser output in a spool of fiberand then recombining the original and delayed laser outputs using asecond optical coupler. The length of the fiber is given by L=ncτ, whereτ is the desired delay time, c is the speed of light and n is therefractive index of the fiber. Using such an optical bufferingapparatus, the effective repetition rate of a laser with an originalduty cycle less than 50% can be doubled. A second optical bufferingstage can be added to quadruple the duty cycle of a swept laser with anoriginal duty cycle less than 25%.

With respect to their commercial use in imaging or spectroscopy systems,the optical buffering configurations that have been disclosed previouslyneed to employ a polarization controller to align the polarizationstates of the original and delayed versions of the laser output if aconsistent sweep-to-sweep polarization state is desired. Onepolarization controller is required for each stage of buffering. Tocompensate for environmental effects on the birefringence of the spooledoptical fiber, a means of adjusting each polarization controllercontinuously to maintain alignment of the polarization states isrequired. These requirements increase system cost and complexity.

Additionally, the optical throughput of such conventional optical bufferconfigurations is relatively low. For example, the optical loss of aconventional buffer, configured for doubling the laser repetition rate,exceeds 3 dB, because at least one-half of the light is lost in thesingle-mode coupler that combines the original and delayed laseroutputs. In certain applications, an optical power loss of thismagnitude can negatively impact a system's signal-to-noise ratio.

The present invention addresses these issues.

SUMMARY OF THE INVENTION

In its various embodiments, the present invention is designed toovercome the main drawbacks—polarization misalignment and high opticallosses—from which conventional optical time-multiplexing methods forlaser sweep-rate multiplication suffer.

In various embodiments, novel optical configurations are introduced thatemploy Faraday mirrors to compensate for alterations of the polarizationstates of the light transmitted through the buffering optics describedin the prior art. In other embodiments, systems and apparatuses forreducing optical losses are disclosed.

In one such embodiment, an optical buffer is combined with aninterferometer in an optical coherence tomography (OCT) system. In otherembodiments of the invention, configurations of one or more opticalelements that reduce losses are described such that couplers inconventional systems are replaced with optical buffers with fast opticalswitches. The states of such buffers are synchronized with an originalportion and a delayed portion of the sweep of the light source. Some ofthe embodiments of the invention include elements or methods steps thateither double or quadruple the optical repetition rate of a tunable orswept light source. However, in some embodiments, further repetitionrate increases can be achieved by extensions or combinations of theconcepts disclosed herein.

In one embodiment, the invention relates to an apparatus for increasingthe repetition rate in a swept light source. The apparatus includes afirst optical coupler having a first arm, a second arm and a third arm;a first mirror in optical communication with the second arm of the firstoptical coupler; and a first optical delay line having a first end inoptical communication with the third arm of the first optical couplerand a second end in optical communication with a second mirror, whereinlight entering the first arm of the first optical coupler leaves thefirst arm of the first optical coupler either delayed by an amount (τ)or substantially undelayed.

In one embodiment, the first and second mirrors are Faraday mirrors. Theapparatus can further include a three port circulator having a firstport, a second port, and a third port. In turn, the first port of thethree port circulator can be configured to receive light from the sweptsource. Further, the second port of the three port circulator can be inoptical communication with the first arm of the first optical coupler.The apparatus can further include an optical isolator in opticalcommunication with the first arm of the first optical coupler. The firstoptical coupler can further include a fourth arm, wherein the apparatusfurther includes an optical isolator in optical communication with thefourth arm of the first optical coupler.

In one embodiment, the first optical coupler further includes a fourtharm; and wherein the apparatus further includes a first polarizationcontroller having a first controller end in optical communication withthe third port of the three port circulator and having a secondcontroller end; a second optical coupler that includes a first arm, asecond arm and a third arm, the first arm of the second optical couplerin optical communication with the second end of the first polarizationcontroller; and a second delay line having a first delay line end inoptical communication with the fourth arm of the first optical couplerand having a second delay line end in optical communication with asecond arm of the second optical coupler. In one embodiment, the secondoptical coupler can include a fourth arm and wherein the apparatusfurther includes an optical switch that includes a first input port, asecond input port and an output port, wherein the first input port ofthe optical switch is in optical communication with the fourth outputarm of the second optical coupler, and wherein the second input port ofthe optical switch is in optical communication with the third arm of thesecond optical coupler.

In one embodiment, the apparatus further includes a second opticalcoupler that includes a first arm, a second arm and a third arm; a thirdmirror in optical communication with the second arm of the secondoptical coupler; and a second optical delay line having a first end inoptical communication with the third arm of the second optical couplerand a second end in optical communication with a fourth mirror; and afour port circulator that includes a first port in optical communicationwith the first arm of the first optical coupler and a second port inoptical communication with the first arm of the second optical coupler;a third port in optical communication with the tunable light source;wherein light entering the first arm of the first optical coupler fromthe first port of the four port circulator, leaves the first arm of thefirst optical coupler and enters the first port of the four portcirculator either delayed by an amount substantially equal to (0) or(τ); and wherein light entering the first arm of the second opticalcoupler from the second port of the four port circulator, leaves thefirst arm of the second optical coupler and enters the second or thirdport of the four port circulator delayed by an amount substantiallyequal to 0, (τ), (τ/2) or (τ/4). In one embodiment, the mirrors areFaraday mirrors.

In one embodiment, the invention relates to an apparatus for increasingthe repetition rate in a tunable light source. The apparatus includes afirst optical coupler that includes a first arm, a second arm and athird arm; a first polarization controller having a first port inoptical communication with the second arm of the first optical couplerand having a second port; a first fiber optic delay line, having a firstend and a second end, the first end of the first fiber delay line inoptical communication with the third arm of the first optical coupler;and an optical switch that includes a first input port, a second inputport and an output port, wherein the first input port of the opticalswitch is in optical communication with the second port of the firstpolarization controller and the second input port of the optical switchis in optical communication with the second end of the first delay line.

In one embodiment, the first optical coupler includes a fourth arm, andwherein the apparatus further includes a second optical coupler thatincludes a first arm, a second arm and a third arm; a secondpolarization controller having a first port in optical communicationwith the first arm of the second optical coupler and having a secondport; and a second fiber optic delay line, having a first end and asecond end, the first end of the second fiber delay line in opticalcommunication with the second arm of the second optical coupler; whereinthe second port of the second polarization controller is in opticalcommunication with the first arm of the first optical coupler, andwherein the second end of the second fiber optic delay line is inoptical communication with the fourth arm of the first optical coupler.

In one embodiment, the invention relates to an optical coherencetomography system. The system includes a first optical coupler thatincludes a first arm, a second arm, a third arm, and a fourth arm; afirst mirror in optical communication with the second arm of the firstoptical coupler; and a first optical delay line having a first end inoptical communication with the third arm of the first optical couplerand a second end in optical communication with a second mirror, whereinlight entering the first arm of the first optical coupler leaves thefirst arm of the first optical coupler either delayed by an amountsubstantially equal to (τ) or substantially undelayed; a four portcirculator, wherein the first port of the four port circulator is inoptical communication with a light source; wherein the second port ofthe four port circulator is in optical communication with the first armof the first optical coupler; a first polarization controller having afirst end in optical communication with the third port of the four portcirculator and having a second end; a second optical coupler thatincludes a first arm, a second arm, a third arm and a fourth arm, thefirst arm of the second optical coupler in optical communication withthe second end of the first polarization controller, the third arm ofthe second optical coupler that includes a sample arm of aninterferometer, the sample arm in optical communication with an OCTprobe, the fourth arm of the second optical coupler in opticalcommunication with a reference mirror, wherein the fourth arm is areference arm of the interferometer; a three port optical circulatorhaving a first port, a second port, and a third port, the second port ofthe three port optical coupler in optical communication with second armof the second optical coupler; a second delay line having a first end inoptical communication with the fourth arm of the first optical couplerand having a second end in optical communication with a second arm ofthe second optical coupler, wherein light entering the second delay lineis delayed by (τ/2); and a pair of first and second balancedphotodetectors, wherein the first photodetector is in opticalcommunication with the fourth port of the four port circulator and thesecond photodetector is in optical communication with the third port ofthe three port circulator.

In one embodiment, the invention relates to an apparatus for increasingthe repetition rate in a tunable light source. The apparatus includes athree port optical circulator; a two by two interferometric switchhaving four ports, the first port of the switch being in opticalcommunication with the second port of the three port circulator; a firstdelay line having a delay of (τ/2) and having a first end in opticalcommunication with the second port of the switch and a second end inoptical communication with a first mirror; a second delay line having adelay of (τ/3) and having a first end in optical communication with thethird port of the switch and a second end in optical communication witha mirror, wherein sweep light leaving the third port of the three portcirculator is delayed by (2τ/3) and (τ/3) relative to sweep lightentering the first port of the three port circulator.

In one embodiment, the invention relates to a method of illuminating asample. The method includes the steps of passing a first sweep pulse oflight through a first delay stage to delay the light by either an amountsubstantially equal to (τ) or 0; passing the first sweep pulse of lightfrom the first delay stage through a second delay stage to delay thelight by a further amount substantially equal to (τ/2) or 0; combiningthe first sweep pulse of light with a second sweep pulse of light andilluminating a sample with the combined first sweep pulse and the secondsweep pulse of light using an optical coherence tomography probe. In oneembodiment, the total delay of light leaving the fourth port of the fourport circulator is substantially equal to at least one of 0, τ/2, τ/4,and τ.

In one embodiment, the invention relates to a method for increasing therepetition rate in a tunable light source. The method includes the stepsof dividing a light sweep pulse into first and second sweep pulses;reflecting the first sweep pulse; passing the second sweep pulse througha first delay line, reflecting the second sweep pulse passed through thefirst delay line back through the delay line to form a delayed secondsweep pulse; and combining the reflected first sweep pulse and thedelayed second sweep pulse. In one embodiment, the method furtherincludes the step of passing the first sweep pulse and the delayedsecond sweep pulse through a second delay line to form a delayed firstsweep pulse and a multiply delayed second sweep pulse; and combining thefirst sweep pulse, the delayed first sweep pulse, a delayed second sweeppulse and a multiply delayed second sweep pulse.

In one embodiment, the invention relates to a method for increasing therepetition rate in a tunable light source. The method includes the stepsof dividing a light sweep pulse into first and second sweep pulses;passing the second sweep pulse through a first delay line to form adelayed second sweep pulse; and multiplexing the first sweep pulse anddelayed second sweep pulse using an optical switch. In one embodiment,the method further includes the steps of dividing the light sweep pulseinto third and fourth sweep pulses; reflecting a third sweep pulse;passing the fourth sweep pulse through a first delay line, reflectingthe fourth sweep pulse passed through the first delay line back throughthe delay line to form a delayed fourth sweep pulse; and combining thereflected first sweep pulse, the delayed fourth sweep pulse and thedelayed second sweep pulse; wherein the third and fourth sweep pulsesare the first and second sweep pulses.

In one embodiment, the present invention enables acquisition ofinterferometric image data at high-repetition rates using opticalbuffering without degradation of image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be understood morecompletely by referring to the drawings described below and theaccompanying descriptions. In the drawings, like numerals are used toindicate like parts throughout the various views.

FIG. 1 is a block diagram showing a laser that includes an opticalbuffer known to the prior art.

FIG. 1A is a graph of a sweep period of the laser shown in FIG. 1 andvarious “on” and “off” states.

FIG. 2 is a block diagram showing one embodiment of an optical bufferthat matches the polarization states of the original and delayedversions of the laser output in accordance with an illustrativeembodiment of the invention.

FIG. 3 is a block diagram showing an optical buffer in which two(optional) optical isolators replace the circulator in the embodiment ofthe optical buffer in FIG. 2 in accordance with an illustrativeembodiment of the invention.

FIG. 4 is a block diagram showing two polarization-aligning opticalbuffers cascaded to enable quadrupling of the laser repetition rate inaccordance with an illustrative embodiment of the invention.

FIG. 4A is a graph depicting the doubling of the sweep repetition rateas a result of the cascaded embodiment of the invention shown in FIG. 4.

FIG. 5 is a block diagram showing two stages that are cascaded to enablequadrupling of the laser repetition rate in accordance with anillustrative embodiment of the invention.

FIG. 6 is a block diagram showing a polarization-aligning buffercombined with a balanced interferometer to quadruple the repetition ratein an optical coherence tomography system, while improving the opticalthroughput in accordance with an illustrative embodiment of theinvention.

FIG. 7 is a block diagram of another embodiment of an optical bufferconfiguration, configured as a repetition-rate doubler, in which one 2×2coupler has been replaced with an optical switch to improve transmissionefficiency in accordance with an illustrative embodiment of theinvention.

FIG. 7A is a graph depicting the sweep repetition rate as a result ofthe switch state of the embodiment of the invention shown in FIG. 7.

FIG. 8 is a block diagram of a switched optical buffer configurationthat enables quadrupling of the repetition rate of a laser sweep inaccordance with an illustrative embodiment of the invention.

FIG. 8A is a graph depicting the quadrupling of the sweep repetitionrate as a result of the switch states of embodiment of the inventionshown in FIG. 8.

FIG. 9 is a block diagram of a switched optical buffer configurationthat enables quadrupling of the repetition rate of a laser sweep inaccordance with an illustrative embodiment of the invention.

FIG. 10 is a block diagram of a polarization-aligning buffer thatincorporates a 2×2 interferometric switch to improve efficiency inaccordance with an illustrative embodiment of the invention.

FIG. 10A is a graph depicting the quadrupling of the sweep repetitionrate as a result of the switch states of the embodiment of the inventionshown in FIG. 10.

DETAILED DESCRIPTION

The following description refers to the accompanying drawings thatillustrate certain embodiments of the invention. Other embodiments arepossible and modifications may be made to the embodiments withoutdeparting from the spirit and scope of the invention. Therefore, thefollowing detailed description is not meant to limit the invention.Rather, the scope of the invention is defined by the appended claims.

Embodiments of the invention describe improved methods, systems, anddevices, such as optical buffers, interferometer switches, and otherelements arranged according to specific configurations for use withvarious data collection and imaging modalities such as optical coherencetomography.

In general, the invention relates to methods and apparatuses forincreasing the repetition rate or rate at which the tunable frequenciesof a swept source can be swept. These methods and apparatuses increasethe repetition rate of a tunable light source by replicating a certainportion of the sweep of the light source. Typically, these light sourcesuse either a piezo-actuated fiberoptic tunable filter or anelectrostatic microelectromechanical (MEMs) tunable filter. Anoscillating, periodic waveform applied to the filter sweeps the opticalfrequency of the laser output back and forth over a spectral range thatdepends on the amplitude and shape of the applied voltage, the dynamicsof the filter movement, and the optical characteristics of the laser.

For most high-speed tunable lasers, the relationship between the appliedvoltage and the emission wavelength is highly nonlinear. Moreover, theoptical characteristics of the laser output during the up-sweep (shortto long wavelength) period usually differ substantially from the opticalcharacteristics of the laser output during the down-sweep (long to shortwavelength) period. The maximum repetition rate of the laser is limitedby the properties of the optical components of the laser, while theeffective duty cycle is limited by the non-linearity and asymmetry ofthe laser emission. For lasers that have an the effective duty cycle ofless than 50%, it is possible to increase the sweep repetition frequencyabove the frequency of the electronic excitation by using an opticalbuffer that time multiplexes the laser output during periods in whichthe laser is normally off In this discussion the light output from theswept laser (or delayed within an optical buffer) will be referred to asthe sweep pulse.

FIG. 1 illustrates an optical buffer configuration from the prior art.The first optical coupler 1 splits the light from the sweep pulse intotwo paths, one that passes through a spool of optical fiber 3 and theother that passes through an adjustable polarization controller 4.Before it recombines with the other path in the second optical coupler2, the sweep pulse that passes through the spool of optical fiber 3 isdelayed by a period of time τ=nL/c, where n and L are the refractiveindex and length of the fiber, respectively, and c is the speed of lightin free space.

L is set to make τ equal to approximately half of a full period of thelaser sweep cycle. As shown in FIG. 1A the laser is turned on onlyduring the useful portion of the sweep period (the interval during whichthe laser produces an output with the desired optical quality). Theoriginal and delayed sweep pulses (the light traveling along the firstpath and the light delayed within the optical fiber, respectively)combine at the output of the second coupler, effectively doubling theeffective sweep repetition rate of the laser.

In this prior-art configuration of the buffer, a polarization controlleris adjusted to align the polarization states of the delayed and originalversions of the laser output. For this configuration, alignment of thepolarization states is required when the doubler is used for OCT imagingsystems and other systems in which a polarization-sensitivesemiconductor amplifier is used after the buffer to boost the laserpower. Polarization alignment avoids or mitigates birefringence-inducedvariations in signal amplitudes in OCT and other interferometric sensingsystems. If the polarization states are misaligned, the relativestrengths of signals acquired on alternate sweeps of the laser can varymarkedly in relation to the birefringence of the sample orbending-induced birefringence of the optical fibers in the sample andreference paths. These signal variations can produce distracting stripeartifacts in OCT images or generate noise in spectrophotometricmeasurements.

FIG. 2 shows an embodiment of the invention in which a swept laser is inoptical communication with an optical buffer that inherently aligns thepolarization states of the time-multiplexed copies of the optical sweeppulse without the need for an adjustable polarization controller. Anoptical circulator 5 directs the sweep pulse from the laser to anoptical coupler 6 that splits the light into two sweep pulses. The firstsweep pulse reflects from a Faraday mirror 9 and passes back through thecoupler 6 and the circulator 5 to the laser output. The second sweeppulse first passes through a delay line or delay coil 8 such as a spoolof fiber, where it is delayed by a time interval (τ/2), before itreflects from a second Faraday mirror 7, passes once again through thespool of fiber 8, and then through the coupler 6 and circulator 5 to thelaser output. That is, both configurations produce two sweeps within aninterval of time (2τ) by delaying the original laser sweep by a lengthof time (τ) and recombining the original and delayed versions of thesweep pulse at the output.

However, in the embodiment of the invention shown in FIG. 2, becauseboth the original and delayed sweep pulses reflect from Faraday mirrors7, 9 and the second sweep pulse passes through the spool 8 twice, thepolarization states of both sweep pulses remain aligned at the output ofthe buffer in spite of birefringence variations in the optical fibersand other components. Since there is no need for active alignment of apolarization controller, the system cost and complexity are reducedwhile reliability and lifetime are increased.

In an alternative configuration of the invention shown in FIG. 3, theoptical circulator 5 (in the embodiment of FIG. 2) is replaced by aninput optical isolator 10 and an output optical isolator 11. In manyapplications, this alternative configuration is just as effective andcan reduce total component costs. If the light source is not sensitiveto optical feedback from light reflected from the optical buffer, theinput isolator 10 is not required. Similarly, if the componentsconnected to the output of the buffer module are not sensitive tooptical feedback, the output isolator 11 is not required. Removing oneor both isolators reduces the number of component in the system and,therefore, reduces system complexity and cost.

The concept of polarization-aligned optical buffering can be extendedfrom doubling to quadrupling of the laser sweep repetition rate.Quadrupling is applicable to lasers with duty cycles less than or equalto 25%. FIG. 4 shows one embodiment of a quadrupling optical buffer inwhich a four-port circulator 13 is used to cascade two doubling stages12, 12′. Each of these stages includes a subset of the elements shown inFIG. 2.

As illustrated by FIGS. 4 and 4A, the first doubling stage 12 combines,in the four port circulator 6, the original sweep pulse from the laserinput with a delayed sweep pulse delayed by an interval of time (τ) dueto the passage of the sweep pulse through the delay line 8, twice. Thesecond doubling stage 12′ delays both of these sweep pulses by aninterval of time (τ/2) and recombines them with the output of the firststage. In this manner, four sweeps appear at the output within therepetition period (2τ). In more detail, light from the four portcirculator 13 is directed toward the first stage 12 where it is treatedas described in the embodiment shown in FIG. 2, forming two sweep pulsesone delayed by (τ) from the other. These two sweep pulses propagate tothe circulator 13 where they are sent to the second stage 12′.

In this second doubling stage 12′, the first sweep pulse, which waspreviously undelayed, is split into two paths by coupler 6′. One pathleads to Faraday mirror 9′. The portion of the previously undelayedsweep pulse traveling this path is reflected by the Faraday mirror 9′back through coupler 6′ to the circulator 13 to the laser output. Thissweep pulse remains undelayed.

The second portion of the previously undelayed sweep pulse split bycoupler 6′ passes through the delay coil of fiber 8′ to Faraday mirror7′ and is reflected back through the delay coil 8′ and through thecoupler 6′ and the four port circulator 13 to the laser output. Becausethe delay coil of fiber 8′ is chosen to introduce a delay of (τ/4)during each pass, the previously undelayed sweep pulse arrives at theoutput with a delay of (τ/2).

The sweep pulse that had been delayed by the first stage 12 by (τ) nextpasses from the four port circulator 13 to the coupler 6′. As shown, aportion of the sweep pulse, previously delayed by (τ/2), is directedtoward mirror 9′. Mirror 9′ reflects the sweep pulse back throughcoupler 6′ through the four port circulator 13 to the laser output. As aresult the sweep pulse which was previously delayed by (τ/2) is nowdelayed by (τ/4). The result is that what was once a single output hasbeen reformed into sweep pulses with delays of 0, τ/4, τ/2, and τrelative to the original laser sweep as shown in FIG. 4A.

FIG. 5 circulator shows an alternative embodiment of a quadruplingoptical buffer in which a buffer, a three-port circulator 5, anddoubling stage 12 (described with respect to FIG. 2) are in opticalcommunication with the output of polarization-aligned optical buffer ofdelay line 8″, polarization controller 4 and coupler 6″. In thisembodiment the sweep pulse from the three-port circulator 5 passesthough a coupler 6 as described in respect to FIG. 2. The resultingsweep pulse that passes back through the coupler 6 to the three portcirculator 5 has delays of 0 and (τ). This sweep pulse passes through apolarization controller 4 before passing to a coupler 6″. The sweeppulse, with delays of 0 and (τ), exit the other arm of coupler 6 andpasses through a delay coil of fiber 8″ which delays that sweep pulse by(τ/2) before passing it to coupler 6″.

This results in sweep pulse light leaving the delay line 8″ having totaldelays of (τ) and (3τ/2). The end result is that light from the sweptsource exits the output with delays of 0, (τ/2), (3τ/2), and τ withrespect to the original laser sweep. The polarization controller 4maintains the polarization states of the two paths.

The minimum optical loss of this buffer is reduced to 3 dB, compared to6 dB for the configuration in FIG. 4. The 3 dB and 6 dB losses referredto here are theoretical minimums based on 50% losses through 1 or 2couplers, respectively. However, the adjustable polarization controlleris required for aligning the polarization states of the optical outputs,thereby increasing the cost.

Although they eliminate the need for a polarization controller to alignthe polarization states of the original and delayed sweeps, the opticalbuffers depicted in FIG. 2-4 have optical losses of (3-7 dB), similar tothose of optical buffers in the prior art. This problem arises from a 3dB loss incurred in each unused output of each optical coupler(generally numeral 6). FIGS. 6-10 show additional embodiments of opticalbuffers with substantially lower optical losses.

In the embodiment of the polarization-aligning buffer in FIG. 5, thefraction of the light from the optical coupler that does not passthrough the optical circulator is lost. In the embodiment shown in FIG.6, optical efficiency is improved by connecting both outputs of thecoupler 6 through optical circulators 13 and 5′ to opposing inputs of abalanced interferometer 14 to form the interferometric portion of an OCTsystem. A second fiber spool or delay line 17 delays one of the inputsto enable a quadrupling of the laser repetition rate. Two photodetectors(15, 16) receive the reference and sample light returning from theinterferometer and generate OCT interference signals with oppositephases. A polarization controller 4 equalizes the polarization states ofthe original and delayed versions of the laser sweeps.

In more detail, the laser input enters the four-port circulator 13 asbefore and the sweep pulse propagates to the coupler 6. Light eitherpasses directly to Faraday mirror 9, or through a delay line 8 to mirror7 before being reflected back to the coupler as discussed with respectto FIG. 2. The difference is that light that would normally be lost tothe unused arm of the coupler 6 is instead passed through a second delayline 17 which imposes an additional delay of (τ/2) before passing thesweep pulse light to a three port circulator 5′. Light from coupler 6 isthen delayed (τ/2), (3τ/2) or (τ). Light from one arm of the three portcirculator 5′ then is sent to a coupler 6″′.

The remaining light from coupler 6 which has been delayed either (τ/2)or 0, passes back to the four-port circulator 13 and to a polarizationcontroller 4 before entering the coupler 6″′. Sweep pulse light thatpropagates to coupler 6″′ from circulator 13 is then combined with sweeppulse light from circulator 5′ before being passed the sample andreference arms of the interferometer 14. This light has delays of 0,(τ/2), (3τ/2) or (τ) relative to the original laser sweep. Sweep pulselight from the reference arm or sample arm is then returned to thecoupler 6″′ and sent to circulators 5′ and 13, which in turn direct thelight to balanced photodetectors 15 and 16.

FIG. 7 shows a further aspect of the invention that includes ahigh-speed 1×2 optical switch 18 to improve optical throughput. Asillustrated in FIG. 7A, the position of the switch (‘A’ or ‘B’) issynchronized with the periods during which the original and delayedversions of the laser sweep that propagate to the output of the opticalcoupler 1. The switch 18 routes the available laser power stored in thedelay line 3 to the buffer output during the period when the laser isturned off. As a result no laser power is lost, except for the excesslosses in the delay line 3 and switch 18. Both electro-optic and MEMsoptical switches that are suitable for high-repetition optical bufferingaccording to this embodiment are available with switching times as shortas about 300 ns and insertion losses less than about 1 dB.

The concept of switch-mode optical buffering can be extended fromrepetition-rate doubling to repetition-rate quadrupling by using theembodiment shown in FIG. 8. This embodiment, which is similar to that ofFIG. 7, adds an additional coupler 20, and an additional twopolarization controllers 4 a, 4 b and a delay line 21 that adds anadditional delay of (τ/2). Only a small optical loss (typically lessthan 1 dB) is incurred as a result of excess losses in a second opticalcoupler 20 and a second delay line 21, that are added to thesingle-stage switched buffer. The optical switch 18, switching between Aand B allows light to go from the polarization controller 4 b or thesecond delay line 21 to the output. Synchronization of the dual-stageswitched buffer is shown in FIG. 8A. Light that propagates to the laseroutput is delayed by 0, (τ), (τ/2) and (3τ/2) relative to the originalsweep of the laser.

The benefits of both aspects of the present invention—polarizationalignment and switch-mode coupling—are realized in the embodimentsdepicted in FIGS. 9 and 10. In the embodiment shown in FIG. 9, theembodiment shown in FIG. 5 is again used but with the unused arm ofcoupler 6″ in FIG. 5, shown as coupler 6″′ in FIG. 9, being connected toone pole of the optical switch 18. Light that would normally be lost bycoupler 6″ in FIG. 5 is switched by high speed switch 18 to the laseroutput. Thus, in this embodiment, polarization-aligned and switch-modebuffering stages are connected in series to allow high-efficiencyquadrupling of the laser repetition rate, using only a singlepolarization controller 4.

In yet another embodiment of the invention, as shown in FIG. 10, nopolarization controllers are required and high efficiency is achievedwith a high-speed 2×2 interferometric switch 23. Employed in high-speedtelecom networks, high-speed 2×2 lithium-niobate interferometricswitches can be made with nanosecond switching times and relatively lowexcess losses. These switches function analogously to a 2×2 splitterwith an arbitrarily adjustable coupling ratio. In this embodiment lightfrom the three port circulator 5 is switched to Faraday mirrors 26 and25 through delay lines 19 and 24 respectively.

FIG. 10A depicts the operating states of the high-efficiency switchedoptical buffer of FIG. 10. During the period when the laser is turnedon, the switch 23 performs the function of a 1×2 coupler, splitting thelaser input received at port B into two equal-intensity sweep pulses. Afirst sweep pulse travels through a delay line 19 (τ/6), and reflectsfrom a first Faraday mirror 26. The sweep pulse then travels back againthrough the delay line 19 for another delay of (τ/6). After this totaldelay of (2τ/3), the switch 23 is reconfigured to route light directlyfrom port A to port B, thereby passing the second sweep pulse to theoptical circulator 5 and then to the laser output of the buffer.

A second sweep pulse travels through a delay line 24 (τ/3), reflectsfrom a second Faraday mirror 25, and travels back again through thedelay line 24 (τ/3). After this total delay of (τ/3), the switch 23 isreconfigured to route light directly from port C to port B, therebypassing the first reflected sweep pulse to the optical circulator 5 andthen to the laser output of the buffer. This buffer module configurationtherefore produces two sweeps followed by a time (τ/3) where there is nooptical output. This time gap can be used, for example, for dataprocessing or storage.

In the description, the invention is discussed in the context of opticalcoherence tomography; however, these embodiments are not intended to belimiting and those skilled in the art will appreciate that the inventioncan also be used for other imaging and diagnostic modalities,instruments for interferometric sensing, or optical systems in general.

The terms light and electromagnetic radiation are used interchangeablyherein such that each term includes all wavelength (and frequency)ranges and individual wavelengths (and frequencies) in theelectromagnetic spectrum. Similarly, the terms device and apparatus arealso used interchangeably. In part, embodiments of the invention relateto or include, without limitation: sources of electromagnetic radiationand components thereof; systems, subsystems, and apparatuses thatinclude such sources; mechanical, optical, electrical and other suitabledevices that can be used as part of or in communication with theforegoing; and methods relating to each of the forgoing. Accordingly, asource of electromagnetic radiation can include any apparatus, matter,system, or combination of devices that emits, re-emits, transmits,radiates or otherwise generates light of one or more wavelengths orfrequencies.

One example of a source of electromagnetic radiation is a laser. A laseris a device or system that produces or amplifies light by the process ofstimulated emission of radiation. Although the types and variations inlaser design are too extensive to recite and continue to evolve, somenon-limiting examples of lasers suitable for use in embodiments of theinvention can include tunable lasers (sometimes referred to as sweptsource lasers), superluminescent diodes, laser diodes, semiconductorlasers, mode-locked lasers, gas lasers, fiber lasers, solid-statelasers, waveguide lasers, laser amplifiers (sometimes referred to asoptical amplifiers), laser oscillators, and amplified spontaneousemission lasers (sometimes referred to as mirrorless lasers orsuperradiant lasers).

The aspects, embodiments, features, and examples of the invention are tobe considered illustrative in all respects and are not intended to limitthe invention, the scope of which is defined only by the claims. Otherembodiments, modifications, and usages will be apparent to those skilledin the art without departing from the spirit and scope of the claimedinvention.

The use of headings and sections in the application is not meant tolimit the invention; each section can apply to any aspect, embodiment,or feature of the invention.

Throughout the application, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited process steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes,” “including,” “have,” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. Moreover, the singular forms “a,”“an,” and “the” include plural forms unless the context clearly dictatesotherwise. In addition, where the use of the term “about” is before aquantitative value, the present teachings also include the specificquantitative value itself, unless specifically stated otherwise.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

Where a range or list of values is provided, each intervening valuebetween the upper and lower limits of that range or list of values isindividually contemplated and is encompassed within the invention as ifeach value were specifically enumerated herein. In addition, smallerranges between and including the upper and lower limits of a given rangeare contemplated and encompassed within the invention. The listing ofexemplary values or ranges is not a disclaimer of other values or rangesbetween and including the upper and lower limits of a given range.

It is to be understood that the figures and descriptions of theinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the invention, while eliminating, forpurposes of clarity, other elements. Those of ordinary skill in the artwill recognize, however, that these and other elements may be desirable.However, because such elements are well known in the art, and becausethey do not facilitate a better understanding of the invention, adiscussion of such elements is not provided herein. It should beappreciated that the figures are presented for illustrative purposes andnot as construction drawings. Omitted details and modifications oralternative embodiments are within the purview of persons of ordinaryskill in the art.

It can be appreciated that, in certain aspects of the invention, asingle component may be replaced by multiple components, and multiplecomponents may be replaced by a single component, to provide an elementor structure or to perform a given function or functions. Except wheresuch substitution would not be operative to practice certain embodimentsof the invention, such substitution is considered within the scope ofthe invention.

The examples presented herein are intended to illustrate potential andspecific implementations of the invention. It can be appreciated thatthe examples are intended primarily for purposes of illustration of theinvention for those skilled in the art. There may be variations to thesediagrams or the operations described herein without departing from thespirit of the invention. For instance, in certain cases, method steps oroperations may be performed or executed in differing order, oroperations may be added, deleted or modified.

Furthermore, whereas particular embodiments of the invention have beendescribed herein for the purpose of illustrating the invention and notfor the purpose of limiting the same, it will be appreciated by those ofordinary skill in the art that numerous variations of the details,materials and arrangement of elements, steps, structures, and/or partsmay be made within the principle and scope of the invention withoutdeparting from the invention as described in the claims.

What is claimed is:
 1. An apparatus for increasing the repetition rate in a swept light source comprising: a first optical coupler comprising a first arm, a second arm and a third arm; a first mirror in optical communication with the second arm of the first optical coupler; and a first optical delay line having a first end in optical communication with the third arm of the first optical coupler and a second end in optical communication with a second mirror, wherein light entering the first arm of the first optical coupler leaves the first arm of the first optical coupler either delayed by an amount (τ) or substantially undelayed.
 2. The apparatus of claim 1 wherein the first and second mirrors are Faraday mirrors.
 3. The apparatus of claim 1 further comprising a three port circulator having a first port, a second port, and a third port, wherein the first port of the three port circulator is configured to receive light from the swept source; and wherein the second port of the three port circulator is in optical communication with the first arm of the first optical coupler.
 4. The apparatus of claim 1 further comprising an optical isolator in optical communication with the first arm of the first optical coupler.
 5. The apparatus of claim 1 wherein the first optical coupler further comprises a fourth arm and wherein the apparatus further comprises an optical isolator in optical communication with the fourth arm of the first optical coupler.
 6. The apparatus of claim 3, wherein the first optical coupler further comprises a fourth arm; and wherein the apparatus further comprises: a first polarization controller having a first controller end in optical communication with the third port of the three port circulator and having a second controller end; a second optical coupler comprising a first arm, a second arm and a third arm, the first arm of the second optical coupler in optical communication with the second end of the first polarization controller; and a second delay line having a first delay line end in optical communication with the fourth arm of the first optical coupler and having a second delay line end in optical communication with a second arm of the second optical coupler.
 7. The apparatus of claim 6 wherein the second optical coupler comprises a fourth arm and wherein the apparatus further comprises an optical switch comprising a first input port, a second input port and an output port, wherein the first input port of the optical switch is in optical communication with the fourth output arm of the second optical coupler, and wherein the second input port of the optical switch is in optical communication with the third arm of the second optical coupler.
 8. The apparatus of claim 1 further comprising a second optical coupler comprising a first arm, a second arm and a third arm; a third mirror in optical communication with the second arm of the second optical coupler; and a second optical delay line having a first end in optical communication with the third arm of the second optical coupler and a second end in optical communication with a fourth mirror; and a four port circulator comprising a first port in optical communication with the first arm of the first optical coupler and a second port in optical communication with the first arm of the second optical coupler; a third port in optical communication with the tunable light source; wherein light entering the first arm of the first optical coupler from the first port of the four port circulator, leaves the first arm of the first optical coupler and enters the first port of the four port circulator either delayed by an amount substantially equal to (0) or (τ); and wherein light entering the first arm of the second optical coupler from the second port of the four port circulator, leaves the first arm of the second optical coupler and enters the third port of the four port circulator delayed by an amount substantially equal to 0, (τ), (τ/2) or (τ/4).
 9. The apparatus of claim 8 wherein the mirrors are Faraday mirrors.
 10. An apparatus for increasing the repetition rate in a tunable light source comprising: a first optical coupler comprising a first arm, a second arm and a third arm; a first polarization controller having a first port in optical communication with the second arm of the first optical coupler and having a second port; a first fiber optic delay line, having a first end and a second end, the first end of the first fiber delay line in optical communication with the third arm of the first optical coupler; and an optical switch comprising a first input port, a second input port and an output port, wherein the first input port of the optical switch is in optical communication with the second port of the first polarization controller and the second input port of the optical switch is in optical communication with the second end of the first delay line.
 11. The apparatus of claim 10 wherein the first optical coupler comprises a fourth arm, and wherein the apparatus further comprises: a second optical coupler comprising a first arm, a second arm and a third arm; a second polarization controller having a first port in optical communication with the first arm of the second optical coupler and having a second port; and a second fiber optic delay line, having a first end and a second end, the first end of the second fiber delay line in optical communication with the second arm of the second optical coupler; wherein the second port of the second polarization controller is in optical communication with the first arm of the first optical coupler, and wherein the second end of the second fiber optic delay line is in optical communication with the fourth arm of the first optical coupler.
 12. An optical coherence tomography system comprising: a first optical coupler comprising a first arm, a second arm, a third arm, and a fourth arm; a first mirror in optical communication with the second arm of the first optical coupler; and a first optical delay line having a first end in optical communication with the third arm of the first optical coupler and a second end in optical communication with a second mirror, wherein light entering the first arm of the first optical coupler leaves the first arm of the first optical coupler either delayed by an amount substantially equal to (t) or substantially undelayed; a four port circulator, wherein the first port of the four port circulator is in optical communication with a light source; wherein the second port of the four port circulator is in optical communication with the first arm of the first optical coupler; a first polarization controller having a first end in optical communication with the third port of the four port circulator and having a second end; a second optical coupler comprising a first arm, a second arm, a third arm and a fourth arm, the first arm of the second optical coupler in optical communication with the second end of the first polarization controller, the third arm of the second optical coupler comprising a sample arm of an interferometer, the sample arm in optical communication with an OCT probe, the fourth arm of the second optical coupler in optical communication with a reference mirror, where in the fourth arm is a reference arm of the interferometer; a three port optical circulator having a first port, a second port, and a third port, the second port of the three port optical coupler in optical communication with second arm of the second optical coupler; a second delay line having a first end in optical communication with the fourth arm of the first optical coupler and having a second end in optical communication with a second arm of the second optical coupler, wherein light entering the second delay line is delayed by (τ/2); and a pair of first and second balanced photodetectors, wherein the first photodetector is in optical communication with the fourth port of the four port circulator and the second photodetector is in optical communication with the third port of the three port circulator. 13.-17. (canceled)
 18. A method for increasing the repetition rate in a tunable light source comprising the steps of: dividing a light sweep pulse into first and second sweep pulses; passing the second sweep pulse through a first delay line to form a delayed second sweep pulse; and multiplexing the first sweep pulse and delayed second sweep pulse using an optical switch.
 19. The method of claim 18 further comprising the steps of: dividing the light sweep pulse into third and fourth sweep pulses; reflecting a third sweep pulse; passing the fourth sweep pulse through a first delay line, reflecting the fourth sweep pulse passed through the first delay line back through the delay line to form a delayed fourth sweep pulse; and combining the reflected first sweep pulse, the delayed fourth sweep pulse and the delayed second sweep pulse, wherein the third and fourth sweep pulses are the first and second sweep pulses.
 20. The method of claim 16 further comprising the step of illuminating a sample using an optical coherence tomography probe using the combined reflected first sweep pulse and delayed second sweep pulse. 